Polyester/starch paper sizing

ABSTRACT

Polyester of trimellitic acid compound having an average molecular weight under about 4,000 and an acid number of at least 35 and starch paper sizing composition containing said polyester.

This invention relates to paper sizing compositions comprising ahydrophilic natural polymer and a polyester of a polyhydric alcohol anda trimellitic acid compound. More particularly, this invention relatesto paper sizing compositions comprising starch and a polyester of apolyhydric alcohol and a trimellitic acid compound.

Paper is often surface-sized with a sizing agent. The sizing agentcements the cellulosic fibers to the body of the paper and to eachother, thereby increasing the tensile strength and Mullen value, orburst strength of the paper. Accordingly, the paper can be erasedwithout destroying its fiber structure. Surface sizing also gives papera relatively smooth, hard film which reduces "feathering" when writtenupon with pen and ink, will not catch the pen point (or pencil point)when paper is written upon and will not pick if the paper is printedwith tacky inks. In addition, the porosity of the paper is decreasedsince the surface pores of the paper are sealed. Surface sizing is oftenmore important than internal sizing for writing papers, printing papersand certain grades of wrapping paper.

Starch, particularly alkaline-hypochorite-oxidized starch and low D.S.derivatives of starch, such as hydroxyethyl starch ethers and cyanoethylstarch ethers, are the principal surface sizing agents used. While thesenatural polymers are effective in improving the aforesaid surfaceproperties of paper, paper sized with these agents is often more watersensitive than desired for some uses, particularly for off-set printing,etc. For these uses, it is often necessary to employ mixtures of variouslatices with the starch to improve the water resistance of thestarch-sized surface.

The principal object of this invention is to provide a new surfacesizing composition for paper. A more specific object of this inventionis to provide a new starch surface sizing composition suitable forapplication to paper at the size press. Other objects appearhereinafter.

We have now found that the hold-out or water-resistance of paper sizingagents, particularly starch surface sizing agents can be improvedmarkedly by employing surface sizing compositions comprising a naturalpolymer and water-soluble polyester of a polyhydric alcohol andtrimellitic acid compound wherein the polyester has an acid number of atleast 35. Applicants believe that the improved surface sizing propertiesof the starch compositions of this invention is due to the interactionof the polyester having a high acid number with the starch resulting incross-linking, or further insolubilization of the hydroxyl groups in thestarch.

The polyesters useful in this invention are substantially linear andhave an average molecular weight under about 4,000. For the most part,these polyesters are of two types. One type contains primarilyintralinear pendant-free carboxyl groups while the other containsprimarily terminalfree carboxyl and/or anhydride groups. The simplesttype of polyester useful in this invention may be viewed as beingsubstantially linear containing intralinear pendant-free carboxylmoieties provided by the trimellitic acid compound. In general, thisfirst class of polyester can be produced by esterifying the trimelliticacid compound (acid or anhydride) with suitable co-monomers (polyhydricalcohols, fatty acids, dicarboxylic acids, etc.) at under 420°F. In thistemperature range, the trimellitic acid compound functions as adicarboxylic acid and substantially each intralinear trimellitic moietyin the polyester has a free caboxyl group.

The second type of polyester is produced by a two-stage cook and can beviewed as having a substantially linear backbone comprising esterifiedpolyhydric alcohol, dicarboxylic acid and possibly some monocarboxylicacid units, having an acid number of about 0 to 25, and sufficientterminal trimellitic groups providing the polyester with an acid numberof at least 35, preferably at least 50. These polyesters must beproduced by a two-step process wherein (1) substantially all of thepolyhydric alcohol moieties (free polyhydric alcohol, polyhydric aocoholesters of monocarboxylic acids, etc.) and carboxylic acid moietieshaving one or two acyl groups are condensed in the first step to producea substantially linear polyester having an acid number of about 0 to 25,and (2) substantially all of the trimellitic acid compound, preferablythe anhydride, is condensed in the second step with the preformedsubstantially linear polyester backbone to produce a polyester having anacid number of at least 35.

The two-stage polyesters, particularly those prepared from trimelliticanhydride, are preferred, since these polyesters contain terminal acidgroups which are apparently responsible for optimum water-resistance ofthe sized paper.

In somewhat greater detail, the first stage of the substantially linearbackbone polyester having an acid number of about 0 to 25 can beproduced by condensing one or more polyhydric alcohol precursors and oneor more carboxylic acid precursors having no more than two acyl groups.The hydroxyl:carboxyl equivalent ratio must be at least 1.1:1 in orderto provide sufficient terminal or internal hydroxyl groups in thesubstantially linear polyester for reaction in the second stage with thetrimellitic acid compound. After the substantially linear polyesterbackbone having an acid number of about 0 to 25 is produced, thetrimellitic acid compound is condensed in the second stage awater-soluble polyester having an acid number of at least 35.

Typically, the backbone polyester is formed by condensing the reactantsat 300° to 500° F. until the polyester has an acid number of about 0 to25 and then the trimellitic acid compound is reacted in the second stageat 250° to 480° F. until a polyester having an acid number of at least35 is formed. For best resuluts, alkyds containing glyceride oilmoieties should have an acid number of at least 55 and those withoutglyceride oil moieties preferably have an acid number of at least 70.

The polyester produced by either the one-stage or two-stage process isgenerally dissolved in water or aqueous medium containing co-solvent oronly co-solvent and/or base prior to blending with natural polymers.Suitable co-solvents include alcohols, such as pentanol,diethyleneglycol monomethyl ether, propylene glycol monopropyl ether,etc. Suitable bases include ammonia, morpholine, alkali metal (sodium orpotassium) hydroxides, etc.

In general, the preferred polyhydric alcohol precursors suitable forproducing the water-soluble polyesters are dihydric alcohols, such asethylene glycol, 1,2-propylene glycol, trimethylene glycol, neopentylglycol, hexamethylene glycol, 4,4'-Bis(beta-hydroxypropyl)-BisphenolA,2,2,4-trimethyl1,3-pentanediol, etc., and the drying or semi-dryingglyceride oils, such as soybean oil, linseed oil, corn oil, etc. Variouspolyhydric alcohols having three or more free hydroxyls, such asglycerol, pentaerythritol, 1,1,1-trimethylol ethane, 1,1,1-trimethylolpropane, 1,2,6-hexanetriol, sorbitol, etc., can be used in relativelylow proportions. To reduce the possibility of rapid molecular weightbuilt up, it is desirable, when using polyhydric alcohols having morethan two hydroxyls, to provide sufficient monocarboxylic acid to providethe polyol with effectively only two free hydroxyl groups per molecule.The dihydric alcohols and polyhydric alcohol esters of monocarboxylicacids having on an average no more than two free-hydroxyl groups permolecular, should constitute at least 90 mole percent of the hydroxylgroups in the polyester backbone to assure that the polyester issubstantially linear.

Suitable dicarboxylic acid compounds useful in this invention includeisophthalic acid, phthalic acid, phthalic anhydride, terephthalic acid,maleic acid, maleic anhydride, fumaric acid, adipic acid, sebacic acid,succinic anhydride, etc.

The trimellitic acid compound (acid or anhydride) should provide fromabout 10 to 100 equivalent percent of the acyl groups in the polyesterwith corresponding 90 to 0 equivalent percent of the acyl groupsprovided by mono and diacyl compounds.

As indicated above, the trimellitic polyesters of this invention can beused to improve the water-resistance of hydrophilic surface sizes.Suitable starch sizes can be based on corn starch, tapioca starch, waxycorn starch, potato starch, wheat starch and the amylopectin fractiontherefrom. Prior to use, these starches are modified, such as byalkaline-hypochlorite-oxidation, acid or enzyme thinning and/orderivatized to a low D.S. (degree of substitution). Suitable derivativesinclude the so-called hydroxyethyl starches, hydroxypropyl starches,cyanoethyl starches, starch acetates, starch propionates, cyanoethylstarch acetates, aminoethyl starches, etc. having a total D.S. of about0.005 to 0.20.

In order to be suitable for use in our invention, it is essential thatthe starch be partially degraded. Partial controlled degradation of thestarch decreases the viscosity of the starch paste used in the sizingoperation, thereby permitting the use of a higher solids sizingcomposition. The controlled degradation of the starch also increases theability of the starch to penetrate the cellulosic fibers andsubsequently to bond the surface fibers together. It is this partialdegradation of the starch which is at least partially responsible forthe relatively poor water-resistance of the sized paper.

There are basically three chemical methods used commercially fordegrading starch, namely, enzyme conversion, oxidation withalkaline-hypochlorite or other oxidation agents and acid hydrolysis. Ingeneral, the starches of this invention are degraded to a fluidity knownin this art of from about 18 to 97 cc, either before or afterderivatization, if derivatization is employed. The method of measuringthe fluidity is explained below. At this point, it is only necessary tounderstand that the higher the fluidity of the starch, the greater thedegree of degradation of the starch. In order to impart to the papermaximum tensile strength and Mullen burst value when sizing at the sizepress, it is necessary to use a starch having a fluidity of 75 to 97 cc.A starch having this fluidity (75 to 97 cc) is most suitable forcementing cellulosic fibers to the body of the paper and to each other.Accordingly, the sized paper is more resistant to tearing of fibersduring erasing. Further, the higher the fluidity of the starch, thehigher the total solids that can be used in the sizing bath. This isparticularly important when using the size press. When sizing at thecalender stack, less highly degraded (lower fluidity) starches can beused with advantage in order to form a less porous sheet.

The alkaline fluidity of degraded granular starches can be determined inthe following manner. Five grams of degraded granular starch (dry solidsbasis) is placed in a 400 millimeter fluidity beaker containingapproximately 100 ccs of starch paste. The composition can be preparedby adding 90 millimeters of 0.25 N sodium hydroxide and 10 millimetersof water to the beaker and the mixture is stirred between 450 and 460r.p.m. for three minutes. The starch paste is poured into a standardfluidity funnel for a specific "water-time" between about 30 and 40seconds. The "water-time" is the number of seconds which takes for 100ccs of water to flow through the funnel. The number of ccs of starchpaste which flows through in the water-time is the fluidity of thestarch. Typically, undegraded starch has about 1 cc fluidity. Thealkaline fluidity of pregelatinized starches or starch pastes can bedetermined in essentially the same manner by adjusting the solidscontent and strength of Na OH to produce a 5 percent solids pastecontaining 0.0225 equivalents of Na OH.

Just prior to use, the granular or pregelatinized starch is slurred inwater at the desired concentration (1 to 20 percent by weight). Thestarch is pasted either by batch means or in a continuous starch cooker(e.g., Votator) at a neutral pH (6 to 8) and then the paste isdischarged into the size box and mixed with the trimellitic polyester. Aweight ratio of 0.1 to 100 parts by weight, preferably 0.5 to 10 partsby weight, trimellitic polyester solids per each 100 parts by weightstarch solids can be used. For the most part, it is desirable to use aslow a concentration of polyester as possible. When a size press isemployed, the size is applied to the paper while the cellulosic web ismoving at 50 to 2,000 feet per minute and then the paper is passed intothe nip between two rolls. The size can be applied to the paper bypassing the paper through a "puddle" of size maintained by supply fromthe size box, passing the paper over one roll of the size presspartially submersed in size solution, by spraying the size compositiononto one or both sides of the paper or by passing the paper web throughthe size bath.

In somewhat greater detail, the size press consists of two rolls betweenwhich the paper travels as it receives the surface sizing solution. In avertical size press, there are two rolls positioned one above the other.Usually size is sprayed onto the bottom roll of the size press orapplied by the bottom roll of the press revolving in a solution of thesize. At the same time, a "puddle" of size solution is supplied to thetop of the roll, usually by spraying. Normally, a spring roll is locatedahead of the press to keep the tension constant and to control the angleat which the paper enters the nip of the press rolls thereby determiningthe area of the "puddle" supported on the top side of the sheet. Theangle is usually about 15°-35°. Lowering the angle increases the area ofthe "puddle," thereby increasing pick up of the size. On the other hand,raising the angle decreases the area of the "puddle" and the pick up ofsize by the cellulosic web.

In a horizontal size press, the two rolls are positioned side by side.The size is normally introduced in the nip at the center of the sheetand flows from the center of the sheet toward both ends. Usually each ofthe two "puddles" formed between one roll surface of the size press andone side of the cellulosic web is kept just large enough to provide alittle size running off the ends of the rolls. During the sizingoperations, the pH of the size solution can drop to as low as four dueto the leaking out of alum from the paper.

Irrespective of how the size is applied at the size press, thecellulosic web passes between rolls in order to drive the size into thepaper and to remove excess size. The nip of the size press rolls isadjusted to exert a pressure of 5 to 200 pounds per linear inch. In thisway, the penetration of the size into the paper is adjusted to meet therequirements of the paper being sized. Usually, at least one of the sizepress rolls is non-resilient while the other roll can be resilient(rubber) or non-resilient. The paper is then dried by suitable means.Generally, the water-resistance of the sized paper of this inventiondevelops to a maximum after aging for about a week or two weeks.

If desired, the size composition can be applied to the cellulosic webusing an air knife, a trailing blade coater, a Champion knife coater, acalender stack, etc.

The following examples are merely illustrative and should not beconstrued as limiting the scope of our invention.

EXAMPLE 1

A trimellitic polyester suitable for use in this invention was preparedby adding 652.6 grams isophthalic acid and 361 grams 1,2-propyleneglycol to a 2-liter kettle equipped with reflux condenser, thermometersand nitrogen sparge. The composition was heated to 360°F. over a periodof about 2 hours. The reactor pot temperature was maintained at about360° to 480° F. for an additional 12 hours while maintaining the refluxhead temperature at below 100° C. to minimize propylene glycol loss andmaximize water removal. After the polyester, having an acid number of6.9, was cooled to 350° F., 125.8 grams trimellitic anhydride was addedto the reactor and the reactor temperature was raised from 350° F. to420° F. over a period of 1 hour to provide a polyester having an acidnumber of 77.2. The polyester was cooled to room temperature anddissolved in Propasol P(1,2-propylene glycol monopropylether) to form a70 percent solids solution, neutralized to pH 9 with concentratedammonium hydroxide and then reduced to 25 percent solids with water.

A 71/2 percent solids aqueous starch composition was produced by pasting50 grams (dry solids basis) granular Penford 280 (a hydroxyethyl starchproduced by reacting 2 percent by weight ethylene oxide with starchhaving a fluidity of 80) with 667 grams water on a stream bath. Thestarch paste was cooled to 150° F. and mixed with 2.5 grams (dry solidbasis) of the polyester prepared in the preceding paragraph. The sizecomposition was placed in the horizontal size bath station of a Keegancoater. Sufficient size composition was applied to both sides of thepaper at the nips to provide a slight run-off at each end. The sizedpaper was then drum dried at 200° F. on a roll dryer providing a contacttime of about 40 seconds and the paper was conditioned at 50 percentrelative humidity and 73° F. for testing. The paper picked up theequivalent of 100 lbs./ton of starch and 5 lbs./ton of polyester. Paperwas also sized in the same manner except thst the polyester was omittedfrom the size bath.

The water resistance of the paper samples was tested on a HerculesPhotosize Tester using a pH 2 blue ink. The number of seconds wasrecorded when the reflectance on the backside of the treated paperdropped to 80 percent of the value of the untreated paper. The resultsare set forth below in Table I.

                                      TABLE I                                     __________________________________________________________________________    Sample              Aging of Paper                                                       As is from size                                                                        Post Cured 15 Minutes                                                                      1 Week                                                                              2 Weeks                                           press    at 105°C.                                          __________________________________________________________________________    Hydroxyethyl starch                                                                      26 sec.  32 sec.      30 sec.                                                                              31 sec.                               Hydroxyethyl starch                                                                      65 sec.  75 sec.      89 sec.                                                                             114 sec.                               and polyester                                                                 __________________________________________________________________________

The above Example illustrates that the trimellitic polyester improvesthe water-resistance of starch sized paper and that the water-resistanceincreases as the paper ages.

Essentially, the same results can be obtained using Staysize 109 (a highfluidity acid hydrolyzed cyanoethyl starch) as the starch.

EXAMPLE II

A polyester useful in this invention based on propylene glycol, maleicanhydride and trimellitic anhydride were prepared in the mannerdescribed in Example I. The details are set forth below in Table No. II.

                                      TABLE II                                    __________________________________________________________________________    Example No. II.     III.     IV.   V.        VI.      VII.                    __________________________________________________________________________    Reactants   PG/MA/TMA                                                                             LO/565/TMA                                                                             SO/565/TMA                                                                            SO/TMPD/IPA/TMA                          Mole Ratios 4/1/1.5 2.2/6/5  2.2/6/5 2/4.5/1.3/.5                                                                          2/6.1/1.4/3.8                                                                          2/6.1/1.4/3.8           Acid No.    80      55       56      64      72       72                      Neutralizing Base                                                                         TEA     NH.sub.4 OH                                                                            NH.sub.4 OH                                                                           NH.sub.4 OH                                                                           NH.sub.4 OH                                                                            NH.sub.4 OH             Cosolvent   None    Isopropanol                                                                            Propasol P                                                                            Propasol P                                                                            Propasol P                                                                             Isopropanol             In the above Table                                                                        PG stands for propylene glycol                                                MA stands for maleic anhydride                                                TMA stands for trimellitic anhydride                                          LO stands for linseed oil                                                     565 stands for 4,4'-bis(beta-hydroxypropyl)-Bisphenol A                       SO stands for soybean oil                                                     TMPD stands for 2,2,4-trimethyl-1,3-pentanediol                               IPA stands for isophthalic acid                                               TEA stands for triethyl amine                                     __________________________________________________________________________

EXAMPLE III

An oil modified polyester was prepared by adding 607.5 grams soybean oiland 658.5 grams of 4,4'-Bis(beta-hydroxypropyl)-Bisphenol A (Dow 565) toa 2-liter kettle equipped with thermometer, nitrogen sparge and a shortair cooled reflux condenser. The composition was heated to 400° F.within 1 hour, at which point 0.24 grams of lithium hydroxide was addedas the alcoholysis catalyst. The temperature was raised rapidly to465°-485° F. and held for 2 hours to achieve good alcoholysis (i.e.,ester interchange). The pot temperature was then lowered to 300° F., atwhich point, 960 grams trimellitic anhydride was charged. The reactorpot temperature was then maintained at about 360°-420° F. for anadditional eight hours to provide an oil modified polyester with an acidnumber of 56. The polyester was cooled to room temperature and dissolvedin Propasol P (1,2-propylene glycol monopropylether) to form a 70%solids solution, neutralized to pH 9 with concentrated ammoniumhydroxide and then reduced to 30 percent solids with water.

EXAMPLE IV TO VII

A series of oil modified polyesters were prepared in the mannerdescribed in Example III. The composition of the polyesters are setforth below in Table II.

The polyesters of Examples II-VII were formulated to provide 1.67 gramspolyester per 50 grams starch (dry solids basis) with Penford 280,applied to paper webs at the size press and tested in the mannerdescribed in Example I. The paper picked up the equivalent of 150lbs./ton of starch and 5 lbs./ton of polyester. The Hercules Photosizeresults are set forth below in Table III.

                                      TABLE III                                   __________________________________________________________________________    Sample          Aging of Paper                                                As is from size                                                                      Post Cured 15 Min.                                                                     1 Week     2 Weeks                                                   press    105°C.                                                 __________________________________________________________________________    Hydroxyethyl                                                                         26 sec   32 sec     30 sec                                                                             31 sec                                         starch                                                                       Ex II  51 sec   56 sec     55 sec                                                                             65 sec                                        Ex III 29 sec   NOT RUN    36 sec                                                                             73 sec                                        Ex IV  34 sec   55 sec     52 sec                                                                             94 sec                                        Ex V   35 sec   69 sec     53 sec                                                                             89 sec                                        Ex VI  33 sec   57 sec     52 sec                                                                             85 sec                                        Ex VII 34 sec   62 sec     54 sec                                                                             94 sec                                        __________________________________________________________________________

While this invention is primarily directed to the trimellitic polyestersof this invention and paper-sized with starch compositions containingsaid polyesters, these compositions can be used advantageously aspigment adhesives at 6 to 20 parts by weight per 100 parts by weightpigment (e.g., clay, satin white, etc.).

We claim:
 1. The method of improving the water-resistance of paper whichcomprises sizing said paper at the size press with a compositioncomprising degraded starch and a polyester of polyhydric alcohol and atrimellitic acid compound, said polyester having an average molecularweight under about 4,000 and an acid number of at least 35, wherein saidpolyester is present in a weight ratio of 0.1 to 100 parts by weight pereach 100 parts by weight starch solids.
 2. The process of claim 1wherein said polyester contains primarily intralinear free carboxylgroups.
 3. The process of claim 1 wherein said polyester has asubstantially linear backbone comprising esterified polyhydric alcoholand dicarboxylic acid moieties having an acid number of about 0 to 25and sufficient terminal trimellitic groups providing the polyester withan acid number of at least
 35. 4. The process of claim 3 wherein saidstarch has a fluidity of about 75 to 97 cc.
 5. The process of claim 4wherein said polyester has an acid number of at least 70.